Since a super capacitor uses an electrostatic characteristic, the super capacitor can be charged and discharged almost infinitely and used semi-permanently in comparison to a battery which uses an electrochemical reaction. Also, since the super capacitor has a very high energy charge and discharge velocity, an output density thereof is several tens of times or more that of a battery which uses an electrochemical reaction.
Due to such characteristics of the super capacitor, application fields thereof are gradually expanding throughout industry.
Specifically, in development fields of next generation environmentally friendly vehicles such as electric vehicles (EV), hybrid electric vehicles (HEV), fuel cell vehicles (FCV), or the like, super capacitor are increasingly used as energy buffers.
The super capacitors are used with batteries as auxiliary energy storage devices. That is, the super capacitors are in charge of instantaneous supply and absorption of energy and the batteries are in charge of average energy supply of vehicles, and thus overall efficiency of a vehicle system can be improved, and a lifetime of an energy storage system can be prolonged.
The super capacitors can be largely classified into an electric double-layer capacitor (EDLC) and a hybrid super capacitor which uses an electrochemical oxidation-reduction reaction.
While an electric double layer is generated on a surface of an EDLC to accumulate electric charges, an electric double layer is formed on a surface of an electrode material in a hybrid super capacitor to accumulate electric charges by an oxidation-reduction reaction, and thus the hybrid super capacitor has an advantage of accumulating relatively more energy.
As disclosed in Korean Unexamined Patent Application Publication No. 10-2013-0016610 (Feb. 18, 2013), in a storage module of a conventional capacitor, a plurality of unit modules including a positive electrode, a negative electrode, and a separation membrane are stacked to form an energy storage assembly, a pair of end plates are disposed at outermost sides of the energy storage assembly, and the energy storage assembly is fixed through coupling beams disposed along edges of the end plates.
In the conventional storage module, when the energy storage assembly is fixed through the pair of end plates and the coupling beams, the coupling beams are concentrated on outer edges of the end plates such that a roll force is more severely generated toward the edges in which the coupling beams are located than at a central portion thereof.
Due to a difference between the roll forces, the energy storage assembly disposed inside the capacitor has a problem in that the center portion thereof is bent in a convex shape (a.k.a., a top phenomenon). This problem results in an imbalance in a distance between electrodes stacked in a vertical direction and causes degraded performance.
In addition, when an electrolytic solution is filled inside the capacitor, bubbles generated upon impregnation of the electrolytic solution are concentrated toward the center portion, which has a relatively low pressure, due to the difference between the roll forces according to position and air pockets are formed such that the air pockets act as factors that hinder impregnation of the positive electrode and the negative electrode with the electrolyte solution.
Accordingly, an imbalance in impregnation between the electrodes is caused and a difference in power generation between the electrodes is generated such that there is a problem in that heat generation and aging proceed rapidly.
Since secondary batteries used in the EDLC should operate within a relatively high temperature range and a temperature thereof is increased when the secondary batteries are continuously used in a high speed charge and discharge state, separation membranes used in the secondary batteries are required to have higher heat resistance and thermal stability than those required for a normal separation membrane. Also, the secondary batteries should have excellent battery characteristics such as rapid charging and discharging, high ion conductivity capable of corresponding to a low temperature, and the like.
In this case, the separation membrane is located between the positive electrode and the negative electrode of the battery to insulate the positive electrode from the negative electrode, and provides a path for ion conduction by maintaining the electrolytic solution. In order to block a current when a temperature of the battery becomes excessively high, the separation membrane has a function in which a portion of the separation membrane is melted to block a pore.
When the temperature of the battery is further increased and the separation membrane is melted, a large hole is generated and a short circuit occurs between the positive electrode and the negative electrode. This temperature is called a short circuit temperature, and the separation membrane should generally have a higher short circuit temperature than a low shutdown temperature thereof.
Conventionally, a cellulose-based separation membrane is mainly used as the separation membrane used in the EDLC. However, when the cellulose-based separation membrane is impregnated with the electrolytic solution having fluidity, the cellulose-based separation membrane tends to easily release the permeated electrolytic solution without maintaining the penetrated electrolyte solution.
Accordingly, in the case in which a plurality of unit modules including a positive electrode, a negative electrode, and a separation membrane are stacked to form an energy storage assembly and an electrolytic solution having fluidity is injected thereinto to complete a capacitor, when the separation membrane fails to maintain the electrolytic solution and releases the electrolytic solution, the electrolytic solution is concentrated on a bottom layer relative to layer units due to fluidity.
This causes a problem in that when the plurality of unit modules are sequentially stacked, separation membranes included in unit modules located at a predetermined height or more do not sufficiently contain the electrolytic solution, and thus performance of the capacitor is degraded.
Meanwhile, in the storage module of the conventional capacitor, a plurality of unit modules including a positive electrode, a negative electrode, and a separation membrane are stacked to form an energy storage assembly, a pair of end plates are disposed at outermost sides of the energy storage assembly, and the energy storage assembly is fixed through coupling beams disposed along edges of the end plates.
In a conventional super capacitor, a current collector, a first electrode, a separation membrane, a second electrode, and a current collector are sequentially and repeatedly stacked when assembled, and a process of bringing the members into close contact through a pressing process is repeatedly performed thereon to prevent generation of gaps between the members after the members are stacked.
As described above, in the conventional super capacitor, since the members are sequentially stacked in sheet units, there is a problem in that alignment between the members in the process of stacking the members is broken, and thus the performance thereof is degraded.
In addition, since a pressing process for bringing the members into close contact is repeatedly performed after the members are stacked, the pressing process is performed several tens to hundreds of times on members located at a lower side thereof and the members are deformed by an excessive roll force such that a thickness deviation with members located at an upper side thereof is generated.
The thickness deviation for each position or height has a problem of degrading the uniform performance.